The sections of a telescopic boom are required to slide axially relative to one another smoothly while at the same time not moving relative to one in a transverse direction. To reduce friction and to take up unavoidable gaps between the sections, wear pads made of a suitable material such as nylon, are placed between the sections at the front and rear axial end of each section.
A known design of wear pad assembly is shown in FIGS. 1 to 4 of the accompanying drawings. FIG. 1 is a section through two overlapping sections 12 and 14 of a telescopic boom 10, the inner section 14 being supported by means of four conventional wear pads assemblies. FIG. 2 shows a plan view of a mounting plate 18 of one of the wear pad assemblies in FIG. 1. Each mounting plate 18, shown in plan in FIG. 2, is itself bolted to the outer section 12 of the boom 10 by means of four bolts 20 and a fifth longer bolt 22 passes through the mounting plate 18 and a shim or spacer 24, which is shown in plan in FIG. 3, to engage within a threaded insert 26 in a wear pad 16, the latter being shown in plan in FIG. 4. The wear pad 16 is in this way clamped to mounting plate 18 and the thickness of the spacer or shim 24 is selected to ensure that the inner surface of the wear pad 16 contacts the outer surface of the inner section 14 to take up the clearance between the two sections 12 and 14 while allowing them to slide smoothly relative to one another.
Because of the accumulation of tolerances, one cannot dimension a wear pad to fit precisely in all locations and instead shims of different thickness are required at the different locations. This creates a requirement for many different parts and complicates assembly. Furthermore, as the pads 16 wear down, the clearance cannot be adjusted without dismantling the wear pad assemblies and replacing the shims 24. A still further disadvantage stems from the fact that only the surface area of the wear pad surrounding the insert 26 can be used to engage the outer surface of the inner section 14 of the boom 10, necessitating the use of larger wear pads 16.